In this paper we present an analytical approach to evaluate the symbol error rate (SER) of OFDM systems subject to carrier frequency offset (CFO) and channel estimation error in Rayleigh flat fading and frequency selective fading channels. Based on correct modeling of the correlation between channel estimates and received signals with carrier frequency offset, the symbol error rate can be numerically evaluated by averaging symbol error rate on different subcarriers using an analytical expression of double integrals. The results illustrate that our analysis can approximate the simulative performance very accurately if the power delay profile of fading channels and carrier frequency offset are known.

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This paper' deals with the feasibility of improving synchronization (packet detection and frequency synchronization) of OFDM systems by exploiting transmit and receive diversities. Starting from a performance analysis of SISO synchronization, SIMO, MISO and MIMO synchronization algorithms are proposed and analyzed step by step. Simulations for IEEE 802.11a/g/n-like systems in frequency-selective channels show that the performance of packet detection can be improved by 6.5 dB and the residual frequency synchronization error can be reduced by 68.7%, if 4 transmit antennas and 4 receive antennas are used.For that, carefully designed training sequences and respective transmitter and receiver algorithms are necessary.

SUMMARYThe performance of the widely applied time-domain channel estimation for SISO-and MIMO-OFDM systems strongly depends on the preciseness of information regarding maximum channel impulse response (CIR) length. In practice the required CIR length is usually approximated by the length of the cyclic prefix (CP) which is an upper bound for most of the actual instantaneous CIR length. In this article, we introduce an appropriate channel length estimation method-named as Frequency Domain Channel Length Indicator (FCLI) which estimates the CIR length minimising the estimated mean square error (MSE) of an instantaneous channel estimate. The simulation results present that the FCLI outperforms the approximation of CP length. In addition, we investigate two applications of FCLI on (1) CP adaptation OFDM systems in which we minimise the overhead of the CP by the estimated CIR length and (2) CP-free OFDM systems in which we directly re-construct the structure of cyclic repetition with reduced error. Compared to the traditional channel estimation with the approximation of CP length, the overall system throughput of either CP adaptation or CP-free OFDM transmission scheme is significantly enhanced regardless of perfect synchronisation or practical distribution of time offsets.

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